At the end of initiation, RNA polymerase undergoes a change in conformation (shape) and is thereafter no longer able to bind to the consensus sequences in the promoter. This change allows the polymerase to escape from the promoter and begin transcribing downstream. The sigma factor is usually released after initiation, although some RNA polymerases may retain sigma throughout elongation.
Transcription takes place within a short stretch of about 18 nucleotides of unwound DNA called the transcription bubble. As it moves downstream along the template, RNA polymerase progressively unwinds the DNA at the leading (downstream) edge of the transcription bubble, joining nucleotides to the RNA molecule according to the sequence on the template, and rewinds the DNA at the trailing (upstream) edge of the bubble.
Transcription is initiated at the start site, which, in bacterial cells, is determined by the binding of RNA polymerase to the consensus sequences of the promoter. No primer is required. Transcription takes place within the transcription bubble. DNA is unwound ahead of the bubble and rewound behind it.